Marine electromagnetic surveying is a geophysical survey technique that uses electromagnetic energy to identify possibly hydrocarbon-bearing rock formations below the bottom of a body of water, such as a lake or ocean. In one example of a marine electromagnetic survey system, an electromagnetic source may be towed at a selected depth in the body of water while a number of electromagnetic sensors are disposed at a substantially stationary position at the water bottom. The survey can be conducted for a range of electromagnetic source and electromagnetic sensor positions. The electromagnetic source can be actuated while towed through the water to emit an electric field that propagates outwardly into the water and downwardly into formations below the water bottom. The electromagnetic sensors detect and record electromagnetic field signals that can be used to infer certain properties of the subsurface rock, such as structure, mineral composition and fluid content, thereby providing information useful in the recovery of hydrocarbons.
Drawbacks to electromagnetic survey systems that use stationary sensors may include limited data acquisition due, for example, to a practical limit on the number of sensors that can be deployed at given time. Additional drawbacks may include the sensitivity of the stationary sensors to variations in the topography of the water bottom. To address these and other drawbacks with stationary sensors, electromagnetic survey systems have been developed that include towing both the electromagnetic source and the electromagnetic sensors through the water. Despite more dense data coverage and less sensor sensitivity to variations on the water bottoms, these towed systems may also have drawbacks. For example, there may be less flexibility in selected source and sensor positions to obtain data on targets from different angles. In addition, towed systems may also have increased induction noise due to movement of both the source and sensors through the water. Also towed systems typically may only measure the inline component of the electric field because the source and sensors may only have electrodes spaced apart in the towing direction.
Accordingly, there is a need for improved methods and systems for marine electromagnetic surveying.